device: rework padding calculation and don't shadow paddedSize

Reported-by: Jayakumar S <jayakumar82.s@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/send.go

@@ -448,6 +448,21 @@ func (peer *Peer) RoutineNonce() {
 	}
 }
 
+func calculatePaddingSize(packetSize, mtu int) int {
+	lastUnit := packetSize
+	if mtu == 0 {
+		return ((lastUnit + PaddingMultiple - 1) & ^(PaddingMultiple - 1)) - lastUnit
+	}
+	if lastUnit > mtu {
+		lastUnit %= mtu
+	}
+	paddedSize := ((lastUnit + PaddingMultiple - 1) & ^(PaddingMultiple - 1))
+	if paddedSize > mtu {
+		paddedSize = mtu
+	}
+	return paddedSize - lastUnit
+}
+
 /* Encrypts the elements in the queue
  * and marks them for sequential consumption (by releasing the mutex)
  *
@@ -514,21 +529,8 @@ func (device *Device) RoutineEncryption() {
 
 			// pad content to multiple of 16
 
-			mtu := int(atomic.LoadInt32(&device.tun.mtu))
+			paddingSize := calculatePaddingSize(len(elem.packet), int(atomic.LoadInt32(&device.tun.mtu)))
-			var paddedSize int
+			for i := 0; i < paddingSize; i++ {
-			if mtu == 0 {
-				paddedSize = (len(elem.packet) + PaddingMultiple - 1) & ^(PaddingMultiple - 1)
-			} else {
-				lastUnit := len(elem.packet)
-				if lastUnit > mtu {
-					lastUnit %= mtu
-				}
-				paddedSize := (lastUnit + PaddingMultiple - 1) & ^(PaddingMultiple - 1)
-				if paddedSize > mtu {
-					paddedSize = mtu
-				}
-			}
-			for i := len(elem.packet); i < paddedSize; i++ {
 				elem.packet = append(elem.packet, 0)
 			}